Recent recovery methods include steam assisted gravity drainage (SAGD) and the solvent co-injection variant thereof. Another method is the so-called N-Solv process.
SAGD (Albahlani, A. M., Babadagli, T., “A Critical review of the Status of SAGD: Where Are We and What is Next?”, SPE 113283, 2008 SPE Western Regional, Bakersfield Calif.) is a method of recovering bitumen and EHO which dates back to the 1960's. A pair of wells is drilled, one above the other. The upper well is used to inject steam, optionally with a solvent. The lower well is used to collect the hot bitumen or EHO and condensed water from the steam. The injected steam forms a chamber that grows within the formation. The steam heats the oil/bitumen and reduces its viscosity so that it can flow into the lower well. Gases thus released rise in the steam chamber, filling the void space left by the oil. Oil and water flow is by a countercurrent gravity driven drainage into the lower well bore. Condensed water and the bitumen or EHO is pumped to the surface. Recovery levels can be as high as 70% to 80%. SAGD is more economic than with the older pressure-driven steam process.
The solvent co-injection variant of the SAGD process (Gupta, S., Gittins, S., Picherack, P., “Insights Into Some Key Issues With Solvent Aided Process”, JCPT, February 2003, Vol 43, No 2) aims to improve the performance of SAGD by introducing hydrocarbon solvent additives to the injected steam. The operating conditions for the solvent co-injection process are similar to SAGD.
In the N-Solv process (Nenniger, J. E., Gunnewiek, L, “Dew Point vs Bubble Point: A Misunderstood Constraint on Gravity Drainage Processes”, CIPC 2009, paper 065; Nenniger, J. E., Dunn, S. G. “How Fast is Solvent Based Gravity Drainage”, CIPC 2008, paper 139), heated solvent vapour is injected into a gravity drainage chamber. Vapour flows from the injection well to the colder perimeter of the chamber where it condenses, delivering heat and fresh solvent directly to the bitumen extraction interface. The N-Solv extraction temperature and pressure are lower than with in situ steam SAGD. The use of solvent is also capable of extracting valuable components in bitumen while leaving high molecular weight coke forming species behind. Condensed solvent and oil then drain by gravity to the bottom of the chamber and are recovered via the production well. Some details of solvent extraction processes are described in CA 2 351 148, CA 2 299 790 and CA 2 552 482.
It is known that contaminants of the solvent injection recovery process may include non-condensable gases, such as carbon dioxide, that may act as a barrier to the process. Methods have been described to remove such gases from the solvent chamber (for example, WO2008/009114).
It is an aim of the present invention to enhance bitumen recovery from a formation and to improve recovery of the injected solvent.